Graphite negative electrode material and preparation method and application thereof

Abstract

The present application provides a graphite negative electrode material and its preparation method and application. The graphite negative electrode material comprises graphite, a hard carbon layer coated on the surface of the graphite, and a metal oxide layer coated on the surface of the hard carbon layer; wherein the graphite negative electrode material satisfies the relationship: d 002 represents the carbon layer spacing of the graphite negative electrode material, ρ represents the compaction density of the graphite negative electrode material, SSA represents the specific surface area of ​​the graphite negative electrode material, T hard represents the average thickness of the hard carbon layer, T metal Represents the average thickness of the metal oxide layer. In this application, the hard carbon layer is a fiber-based hard carbon layer, obtained by carbonizing organic fibers. It bonds evenly with graphite, exhibits good strength, has high interfacial stability, and can reduce the specific surface area of ​​graphite. The metal oxide layer can improve the electronic conductivity and initial coulombic efficiency of the material. Furthermore, the hard carbon layer and the metal oxide layer can work synergistically to effectively improve the rate performance and specific capacity of the graphite negative electrode material.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of lithium ion batteries, and particularly relates to a graphite negative electrode material and a preparation method and application thereof. BACKGROUND

[0002] As an important secondary battery, lithium ion batteries are widely used in the fields of new energy vehicles, portable electronic devices and the like. With the increasing requirements of the market on the energy density and rate performance of lithium ion batteries, the traditional graphite negative electrode material has been difficult to meet the market demand due to its low theoretical specific capacity (372 mAh / g) and poor rate performance.

[0003] At present, the main method for improving the energy density and rate performance of the graphite negative electrode material is to perform surface coating, such as coating with soft carbon or hard carbon. However, the traditional carbon coating layer has problems of uneven coating and poor interface stability, which leads to the reaction of the composite material with the electrolyte, thereby affecting the initial efficiency and rate performance of the battery. For example, the hard carbon coating layer itself has poor conductivity, which leads to an increase in lithium ion diffusion impedance and cannot meet the demand for fast charging. The soft carbon coating layer often needs a high coating amount and has poor coating effect, and does not add capacity to the graphite.

[0004] Therefore, it is urgent to develop a new type of composite coating material to solve the problems existing in the existing coating materials while ensuring high energy density and rate performance.

[0005] The content in the background section is only the technology known to the inventors and does not necessarily represent the existing technology in the field. SUMMARY

[0006] In order to solve at least one of the above technical problems in the prior art, the application provides a graphite negative electrode material and a preparation method thereof.

[0007] The graphite negative electrode material provided by the application comprises graphite, a hard carbon layer coated on the surface of the graphite, and a metal oxide layer coated on the surface of the hard carbon layer.

[0008] The graphite negative electrode material satisfies the following relationship: d 002 represents the carbon layer spacing of the graphite negative electrode material, p represents the compacted density of the graphite negative electrode material, SSA represents the specific surface area of the graphite negative electrode material, T hard represents the average thickness of the hard carbon layer, T metal represents the average thickness of the metal oxide layer.

[0009] In the present application, the hard carbon layer is a fiber-based hard carbon layer, which is obtained by carbonization of organic fibers, uniformly combined with graphite, with good strength, high interface stability, and can reduce the specific surface area of graphite. The metal oxide layer can improve the electronic conductivity and the first coulomb efficiency of the material. Moreover, the hard carbon layer and the metal oxide layer can synergistically improve the rate performance and specific capacity of the graphite negative electrode material.

[0010] In some embodiments of the present application, the graphite negative electrode material satisfies at least one of the following conditions:

[0011] a) the thickness ratio of the hard carbon layer to the metal oxide layer is (1.3-3.6):1;

[0012] b) the average thickness of the hard carbon layer is 60-90 nm;

[0013] c) the average thickness of the metal oxide layer is 25-45 nm;

[0014] d) the hard carbon layer is formed by carbonization of high-performance fibers, and the high-performance fibers are polydopamine polarized high-performance fibers, and the high-performance fibers optionally include at least one or several of polyethylene fibers, polystyrene fibers, and polyimide fibers.

[0015] Under the above conditions, the hard carbon layer has better bonding strength, higher interface stability, and more uniform interface with graphite, and can better reduce the specific surface area of graphite; moreover, the hard carbon layer and the metal oxide layer have the best synergistic effect, which can better improve the electronic conductivity and the first coulomb efficiency of the material.

[0016] In some embodiments of the present application, the graphite negative electrode material satisfies at least one of the following conditions:

[0017] e) the interlayer spacing d 002 of the carbon layer of the graphite negative electrode material is 0.355-0.361 nm;

[0018] f) the specific surface area SSA 2 of the graphite negative electrode material is ≤2.5 m

[0019] g) the compaction density p 3 of the graphite negative electrode material is ≥1.55 g / cm

[0020] h) the average width L a of the graphite crystal in the graphite negative electrode material along the a-axis direction is 75-100 nm;

[0021] i) the average height L c of the graphite crystal in the graphite negative electrode material along the c-axis direction is 25-35 nm.

[0022] Under the above conditions, the performance of the graphite negative electrode material is better.

[0023] The application provides a method for preparing the graphite negative electrode material.

[0024] S1: reacting dopamine with high-performance fibers, so that the dopamine is polymerized on the surface of the high-performance fibers and forms an adhesive film, thereby obtaining an intermediate material;

[0025] S2: mixing the intermediate material with graphite, so that the graphite is coated by the intermediate material, and then pyrolyzing to obtain a graphite precursor;

[0026] S3: immersing the graphite precursor in a metal salt solution, drying, and then performing low-temperature carbonization, thereby obtaining the graphite negative electrode material.

[0027] The application introduces dopamine to form a large number of amino and hydroxyl functional groups on the surface of high-performance fibers, and then coats the graphite, reacts with a metal salt solution, and finally carbonizes to form a hard carbon layer and a metal oxide layer coated on the surface of the graphite, which have a synergistic effect, and can effectively improve the rate performance and specific capacity of the prepared graphite material.

[0028] In some embodiments of the application, step S1 comprises:

[0029] dissolving the dopamine and the high-performance fibers in water, stirring at a temperature of 60-80 DEG C for 2-5 h, so that the dopamine is polymerized on the surface of the high-performance fibers and forms an adhesive film, thereby obtaining an intermediate material;

[0030] Optionally, the high-performance fibers comprise at least one or several of polyethylene fibers, polystyrene fibers and polyimide fibers.

[0031] Optionally, the molecular weight of the high-performance fibers is greater than or equal to 3 x 10 6 g / moL;

[0032] Optionally, the mass ratio of the dopamine to the high-performance fibers is 1:(1-3).

[0033] The dopamine contains a large number of amino and hydroxyl functional groups, and step S1 polarizes the high-performance fibers using polymerized dopamine, so that the dopamine is polymerized on the surface of the high-performance fibers and forms an adhesive film, thereby forming a large number of amino and hydroxyl functional groups on the surface of the fibers and enhancing the surface activity.

[0034] In some embodiments of the application, step S2 comprises:

[0035] dispersing the graphite in a solution with a pH of 2.0-3.0 to obtain a graphite dispersion solution; and

[0036] stirring and mixing the graphite dispersion solution with the intermediate material, so that the graphite is coated by the intermediate material, and then drying and pyrolyzing at 300-500℃ for 2-4h in an inert atmosphere to obtain the graphite precursor;

[0037] Optionally, the average particle size of the graphite is 8-20μm.

[0038] Optionally, the solution is one or more of hydrochloric acid, nitric acid, sulfuric acid and HF.

[0039] Optionally, the stirring and mixing time is 30-90min.

[0040] Optionally, the concentration of the graphite dispersion solution is 0.2-0.4kg / L.

[0041] Optionally, the inert atmosphere is one or more of helium, nitrogen and argon.

[0042] Step S2 coats the graphite with high-performance fibers having an adhesive film formed on the surface, and then pyrolyzes. Since the fiber surface has a large number of amino and hydroxyl functional groups, under the action of electrostatic interaction and covalent bonding, the graphite is combined to improve the bonding strength of the coating layer and the graphite and improve the interface stability. In the process of pyrolysis, the fiber will shrink under heat, part of which will fill in the pores of the graphite, and the rest will coat the surface of the graphite, forming a uniform and stable hard carbon coating layer, thereby reducing the specific surface area of the graphite.

[0043] In some embodiments of the present application, step S3 comprises:

[0044] immersing the graphite precursor in the metal salt solution, drying and then carbonizing at a temperature of 500-700℃ for 1-2h in an inert atmosphere to obtain the graphite negative electrode material.

[0045] Optionally, the metal salt solution is one or more of aluminum nitrate, iron nitrate, ferric chloride and nickel nitrate solution.

[0046] Optionally, the inert atmosphere is one or more of helium, nitrogen and argon.

[0047] The graphite precursor with hydroxyl functional groups in step S3 can combine with metal salt ions to form a stable structure, so that the metal salt is coated on the surface of the graphite precursor, and after drying and low-temperature carbonization, the fiber and metal salt coated on the surface of the graphite are completely carbonized, thereby forming a hard carbon layer and a metal oxide layer. The present application also provides a negative electrode sheet comprising the graphite negative electrode material described above or prepared by the preparation method described above.

[0048] The application further provides a negative electrode sheet comprising the graphite negative electrode material described above or prepared by the preparation method described above.

[0049] The application further provides a lithium ion battery comprising the negative electrode sheet described above.

[0050] In some embodiments of the application, the lithium ion battery has a reversible specific discharge capacity ≥ 375 mAh / g, a first coulombic efficiency ≥ 95%, a capacity retention rate ≥ 90% at 200 cycles under 1C, and a capacity retention rate ≥ 75% under 5C. The lithium ion battery provided by the application has excellent rate performance and specific capacity.

[0051] Additional aspects and advantages of the application will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following description and drawings or can be learned by practice of the application. BRIEF DESCRIPTION OF DRAWINGS

[0052] The accompanying drawings, which are incorporated in and constitute a part of this disclosure, are intended to provide further understanding of the disclosure and are incorporated herein in their entirety. The illustrative embodiments of the disclosure and their description serve to explain the disclosure. They do not, however, limit the disclosure.

[0053] Figure 1 is a process flow chart for preparing the graphite negative electrode material provided by an embodiment of the application.

[0054] Figure 2 is an SEM image of the graphite negative electrode material prepared by an embodiment of the application, with a magnification of 3000x.

[0055] Figure 3 is a partial enlarged view of Figure 2 , with a magnification of 10000x.

[0056] Figure 4 is an SEM image of the graphite negative electrode material prepared by another embodiment of the application, with a magnification of 3000x.

[0057] Figure 5 is a partial enlarged view of Figure 4 , with a magnification of 10000x. DETAILED DESCRIPTION

[0058] In the following, only certain exemplary embodiments are simply described. As those skilled in the art can recognize, the described embodiments can be modified in various different ways without departing from the spirit or scope of the application. Therefore, the drawings and description are considered to be exemplary in nature rather than limiting.

[0059] The disclosure provided below provides many different implementations or examples to implement the present application. In order to simplify the disclosure of the present application, the components and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the present application. In addition, the present application can repeatedly refer to numerals and / or reference letters in different examples, and such repetition is for the purpose of simplification and clarity, which itself does not indicate the relationship between the various embodiments and / or settings discussed. In addition, the present application provides examples of various specific processes and materials, but those of ordinary skill in the art can realize the application of other processes and / or the use of other materials.

[0060] In addition, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present application, and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0061] "About" or "approximately," as used herein when used in connection with a measurement or a value, includes the stated value and means within a reasonable range of error as determined by one of ordinary skill in the art to which the measurement or value pertains. For example, "about" can mean within one or more standard deviations, or within ±30%, ±20%, ±10%, or ±5% of the stated value.

[0062] The specific embodiments of the present application are described in detail below with reference to the accompanying drawings and examples, so that the schemes of the present application and the advantages of various aspects thereof can be better understood. However, the following description of specific embodiments and examples is for illustrative purposes only, and is not a limitation on the present application.

[0063] The graphite negative electrode material provided in the present application is a core-shell structure, in which graphite is the core, and two shell layers are coated on the surface of the core. The inner shell layer is a hard carbon layer, and the outer shell layer is a metal oxide layer. In the present application, the hard carbon layer is a fiber-based hard carbon layer, which is obtained by carbonizing high-performance organic fiber. It is uniformly combined with graphite, has good strength, high interface stability, and can reduce the specific surface area of graphite. The metal oxide layer can improve the electronic conductivity and the first coulombic efficiency of the material. In the present application, the hard carbon layer and the metal oxide layer can synergistically improve the rate performance and specific capacity of the graphite negative electrode material.

[0064] In the present application, the graphite negative electrode material satisfies the relationship: wherein d 002represents the average thickness of the carbon layer of the graphite negative material, p represents the compacted density of the graphite negative material, SSA represents the specific surface area of the graphite negative material, T hard represents the average thickness of the hard carbon layer, T metal represents the average thickness of the metal oxide layer. Within the range of the above parameters, it is shown that the graphite negative material has very good rate performance and specific capacity. In some embodiments of the present application, The value of the ratio of the average thickness of the hard carbon layer to the average thickness of the metal oxide layer can be 0.5, 0.7, 0.9, 1.0, 1.1, 1.3, or 1.5.

[0065] Optionally, the graphite includes natural graphite and / or artificial graphite.

[0066] Optionally, the thickness ratio of the hard carbon layer to the metal oxide layer is (1.3-3.6):1. When within the above thickness ratio range, the hard carbon layer and the metal oxide layer have the best synergistic effect. In some embodiments of the present application, the thickness ratio can be 1.3:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.3:1, or 3.6:1.

[0067] Optionally, the average thickness of the hard carbon layer is 60-90 nm. When the hard carbon layer is within this range, the hard carbon layer has better bonding strength with the graphite, higher interface stability, more uniform interface, and can better reduce the specific surface area of the graphite. In some embodiments of the present application, the average thickness of the hard carbon layer can be 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, or 90 mm.

[0068] Optionally, the average thickness of the metal oxide layer is 25-45 nm. When the thickness of the metal oxide layer is within this range, it can better improve the electronic conductivity and the first coulombic efficiency of the material. In some embodiments of the present application, the average thickness of the metal oxide layer can be 25 nm, 30 nm, 35 nm, 40 nm, or 45 nm.

[0069] Optionally, the hard carbon layer is formed by carbonization of high-performance fibers, and the high-performance fibers are fibers after polarization treatment of polydopamine. In the present application, high-performance fibers refer to fibers with a strength greater than 17.6 cN / dtex and an elastic modulus greater than 440 cN / dtex. Such fibers have high strength and large elastic modulus, and the performance of the hard carbon formed after carbonization is also very good, which can more effectively improve the energy density and rate performance of the material. Optionally, the high-performance fibers include one or more of polyethylene fibers, polystyrene fibers, and polyimide fibers. The high-performance fibers after polarization of polydopamine can form a large number of amino and hydroxyl functional groups on the fiber surface. In the subsequent mixing process with graphite, under the action of electrostatic interaction and covalent bonding, the graphite is tightly combined, which can improve the bonding strength of the hard carbon coating layer with the graphite and increase the thickness of the hard carbon coating layer, and improve the interface stability.

[0070] Optionally, the carbon interlayer spacing d of the graphite negative electrode material is 0.355-0.361 nm. 002 The carbon interlayer spacing is large, and the lithium storage sites are more. In some embodiments of the present application, the carbon interlayer spacing d of the graphite negative electrode material can be 0.355 nm, 0.356 nm, 0.357 nm, 0.358 nm, 0.359 nm, 0.360 nm or 0.361 nm. 002

[0071] Optionally, the specific surface area SSA of the graphite negative electrode material is ≤2.5 m 2 / g. The specific surface area is small, the surface defects are few, and the performance of the material is better. In some specific embodiments, the specific surface area SSA of the graphite negative electrode material can be 2.5 cm 2 / g, 2.3 cm 2 / g, 2.0 cm 2 / g, 1.8 cm 2 / g, 1.6 cm 2 / g, 1.4 cm 2 / g or 1.2 cm 2 / g.

[0072] Optionally, the compaction density p of the graphite negative electrode material is ≥1.55 g / cm 3 The greater the compaction density, the higher the energy density of the lithium ion battery. In some embodiments of the present application, the compaction density p of the graphite negative electrode material can be 1.55 g / cm 3 , 1.57 g / cm 3 , 1.59 g / cm 3 , 1.61 g / cm 3 , 1.63 g / cm 3 , 1.65 g / cm 3 , 1.67 g / cm 3 or 1.69 g / cm 3 .

[0073] Optionally, the average width L a of the graphite crystal in the graphite negative electrode material along the a-axis direction is 75-100 nm. The greater the L a value, the more lithium inserted, and the greater the lithium storage capacity. In some embodiments of the present application, the L a value can be 75 nm, 80 nm, 85 nm, 90 nm, 95 nm or 100 nm.

[0074] Optionally, the average height L c of the graphite crystal in the graphite negative electrode material along the c-axis direction is 25-35 nm. The greater the L c ​The graphite structure is longitudinally stacked well within the above range, and does not have a negative impact on the diffusion and embedding efficiency of lithium ions. In some embodiments of the present application, L c The value can be 25 nm, 27 nm, 29 nm, 31 nm, 33 nm, or 35 nm.

[0075] Figure 1 A preparation method of a graphite negative electrode material provided by an embodiment of the present application is shown, which includes the following steps S1-S3.

[0076] S1: Reacting dopamine with high-performance fibers to make the dopamine polymerize on the surface of the high-performance fibers and form an adhesive film, thereby obtaining an intermediate material.

[0077] The dopamine contains a large number of amino and hydroxyl functional groups. In this step, the high-performance fibers are polarized by the polymerized dopamine, so that the dopamine polymerizes on the surface of the high-performance fibers and forms an adhesive film, thereby forming a large number of amino and hydroxyl functional groups on the surface of the fibers and enhancing the surface activity.

[0078] In some embodiments of the present application, step S1 can specifically be: dissolving dopamine and high-performance fibers in water, stirring at a temperature of 60-80°C for 2-5 h, so that the dopamine polymerizes on the surface of the high-performance fibers and forms an adhesive film, thereby obtaining an intermediate material.

[0079] The mild stirring at 60-80°C ensures the oxidative self-polymerization of the dopamine, thereby forming an adhesive film on the surface of the high-performance fibers, and does not cause other reactions of the fibers themselves. In some embodiments of the present application, the reaction temperature can be 60°C, 65°C, 70°C, 75°C, or 80°C.

[0080] As described above, the high-performance fibers include one or more of polyethylene fibers, polystyrene fibers, and polyimide fibers. Optionally, the molecular weight of the high-performance fibers is ≥ 3 x 10 6 The high-performance fibers with a larger molecular weight, such as polyethylene fibers and polyimide fibers, can better coat the surface of the graphite.

[0081] Optionally, the mass ratio of dopamine to high-performance fibers is 1:(1-3). After the oxidative self-polymerization of the dopamine, the dopamine polymerizes on the surface of the high-performance fibers and forms an adhesive film, thereby forming a large number of amino and hydroxyl functional groups on the surface of the fibers. Within the above mass ratio range, the dopamine is not wasted, and a large number of amino and hydroxyl functional groups can be formed on the surface of the fibers. In some embodiments of the present application, the mass ratio of dopamine to high-performance fibers can be 1:1, 2:3, 1:2, 2:5, or 1:3.

[0082] S2: mixing the intermediate material with the graphite so that the graphite is coated by the intermediate material, and then pyrolyzing to obtain the graphite precursor.

[0083] This step coats the graphite with the high-performance fiber (i.e., the intermediate material) having an adhesive film formed on the surface, and then pyrolyzes. Since the fiber surface has a large number of amino and hydroxyl functional groups, the coating layer can be combined with the graphite under the action of electrostatic interaction and covalent bonding, so that the combination strength of the coating layer with the graphite is improved, and the interface stability is improved. In the process of pyrolysis, the fiber shrinks under heat, and part of it fills in the pores of the graphite, and the rest coats the surface of the graphite to form a uniform and stable hard carbon coating layer, thereby reducing the specific surface area of the graphite.

[0084] In some embodiments of the present application, step S2 can specifically be: dispersing the graphite in a solution with a pH of 2.0-3.0 to obtain a graphite dispersion solution; and stirring and mixing the graphite dispersion solution with the intermediate material so that the graphite is coated by the intermediate material, and then drying and pyrolyzing at 300-500°C in an inert atmosphere for 2-4h to obtain the graphite precursor.

[0085] The graphite is first acidized in an acidic solution with a pH of 2.0-3.0, so that the graphite can be better combined with the high-performance fiber having a large number of amino and hydroxyl functional groups on the surface. Optionally, the solution is one or more of hydrochloric acid, nitric acid, sulfuric acid or HF. In some embodiments of the present application, the pH of the acidic solution can be 2.0, 2.2, 2.4, 2.6, 2.8 or 3.0.

[0086] Optionally, the average particle size of the graphite is 8-20μm. The average particle size of the graphite directly affects the average particle size of the graphite negative electrode material, and in the above particle size range, the performance of the prepared graphite negative electrode material is better. In some embodiments of the present application, the average particle size of the graphite can be 8μm, 10μm, 12μm, 14μm, 16μm, 18μm or 20μm.

[0087] Optionally, the concentration of the graphite dispersion solution is 0.2-0.4kg / L. The concentration in this range has a better acidizing effect on the graphite. In some embodiments of the present application, the concentration of the graphite dispersion solution can be 0.2kg / L, 0.25kg / L, 0.3kg / L, 0.35kg / L or 0.4kg / L.

[0088] Optionally, the stirring and mixing time is 30-90min. Under this stirring time, the graphite is stirred uniformly with the intermediate material so as to be coated by the intermediate material. In some embodiments of the present application, the stirring and mixing time can be 30min, 40min, 50min, 60min, 70min, 80min or 90min. The stirring and mixing can be carried out at room temperature.

[0089] In the present application, the pyrolysis temperature is 300-500℃, and the time is 2-4h. Under this temperature and time, the high-performance fiber coated on the surface of graphite with a large number of amino and hydroxyl functional groups will not be completely carbonized, and still has part of the hydroxyl functional groups. In the process of reacting with the metal salt solution, the hydroxyl functional groups can combine with metal salt ions to form a stable structure. In some embodiments of the present application, the pyrolysis temperature can be 300℃, 350℃, 400℃, 450℃ or 500℃, and the time can be 2h, 2.5h, 3h, 3.5h or 4h.

[0090] Optionally, the drying method is oven drying, vacuum drying or natural drying, etc. Optionally, the inert atmosphere is one or more of helium, nitrogen, argon.

[0091] S3: The graphite precursor is immersed in a metal salt solution, dried, and then low-temperature carbonized to obtain a graphite negative electrode material.

[0092] In this step, the graphite precursor with hydroxyl functional groups can combine with metal salt ions to form a stable structure, so that the metal salt is coated on the surface of the graphite precursor. After drying, low-temperature carbonization is performed, so that the fiber and metal salt coated on the surface of the graphite are completely carbonized, thereby forming a hard carbon layer and a metal oxide layer.

[0093] In some embodiments of the present application, step S3 can specifically be: the graphite precursor is immersed in a metal salt solution, dried, and then carbonized at a temperature of 500-700℃ in an inert atmosphere for 1-2h to obtain a graphite negative electrode material.

[0094] The carbonization temperature of 500-700℃ and the carbonization time of 1-2h can ensure that the fiber and the metal salt are completely carbonized, and also prevent the metal salt from decomposing, so that a hard carbon layer and a metal oxide layer can be obtained. In some embodiments of the present application, the carbonization temperature can be 500℃, 550℃, 600℃, 650℃ or 700℃, and the carbonization time can be 1h, 1.5h or 2h.

[0095] Optionally, the metal salt solution is one or more of aluminum nitrate, iron nitrate, ferric chloride, and nickel nitrate solution. These metal ions are more conducive to improving the performance of the hard carbon negative electrode material.

[0096] Optionally, the drying method is oven drying, vacuum drying or natural drying, etc. Optionally, the inert atmosphere is one or more of helium, nitrogen, argon.

[0097] The present application forms a large number of amino and hydroxyl functional groups on the surface of high-performance fibers by introducing dopamine, then coats graphite, and then reacts with a metal salt solution, and finally carbonizes to form a hard carbon layer and a metal oxide layer coated on the surface of the graphite, which have a synergistic effect, and can effectively improve the rate performance and specific capacity of the prepared graphite material.

[0098] The present application further provides a negative electrode sheet comprising the above-mentioned graphite negative electrode material or the graphite negative electrode material prepared by the above-mentioned preparation method.

[0099] The negative electrode sheet generally comprises a negative electrode current collector and a negative electrode film layer arranged on the negative electrode current collector, wherein the negative electrode film layer can comprise the above-mentioned graphite negative electrode material.

[0100] The current collector can be a metal foil, such as an aluminum foil, a copper foil, etc., preferably a copper foil. The negative electrode film layer can further comprise a binder, a conductive agent, etc. The binder can be, for example, styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), sodium carboxymethyl cellulose (CMC-Na), sodium alginate, etc. The conductive agent can be, for example, graphene, carbon nanotubes, Ketjen black, conductive carbon black (SP), etc. Optionally, the negative electrode film layer can further comprise other auxiliary agents, such as a dispersing agent (for example, carboxymethyl cellulose (CMC)), etc.

[0101] The present application further provides a lithium ion battery comprising the above-mentioned negative electrode sheet. The lithium ion battery further comprises a positive electrode sheet, a separator and an electrolyte. The lithium ion battery provided by the present application has very good reversible discharge specific capacity, first coulombic efficiency and capacity retention rate. Optionally, the reversible discharge specific capacity of the lithium ion battery is ≥375 mAh / g. Optionally, the first coulombic efficiency of the lithium ion battery is ≥95%. Optionally, the capacity retention rate of the lithium ion battery at 1C for 200 cycles is ≥90%. Optionally, the capacity retention rate of the lithium ion battery at 5C is ≥75%.

[0102] The lithium ion battery of the present application is a secondary battery, which refers to a battery that can be used continuously by activating the active material through charging after discharging. Generally, a secondary battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. During the charging and discharging process of the battery, active ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet and plays a role of isolation. The electrolyte plays a role of ion conduction between the positive electrode sheet and the negative electrode sheet.

[0103] The application will be described in the following with reference to specific examples. The values of the process conditions taken in the following examples and comparative examples are exemplary, and the values of the ranges thereof are as indicated in the foregoing summary of the application. For the process parameters not specifically mentioned, the conventional techniques can be referred to. Unless specifically indicated, the reagents and instruments used in the technical solutions provided by the application can be purchased from the conventional channels or market. It should be noted that, in the case of no conflict, the examples in the present application and the features in the examples can be combined with each other.

[0104] Example 1

[0105] In this example, a graphite negative electrode material is prepared, and the specific steps are as follows:

[0106] 1) Dopamine and polyethylene fibers with a molecular weight of 3 x 10 6 g / moL are mixed according to a mass ratio of 1:2 and dissolved in 200 mL of deionized water, stirred at a temperature of 60°C for 4 h, and an intermediate material is obtained;

[0107] 2) 50 g of natural flake graphite is dispersed in 200 mL of hydrochloric acid solution with a pH value of 2.0, then mixed with the intermediate material according to a ratio of 1:1, vacuum dried, and heated to 300°C at a rate of 3°C / min in a nitrogen atmosphere, pyrolyzed for 4 h to form a hard carbon layer, and a graphite precursor is obtained;

[0108] 3) The graphite precursor is immersed in a mixed solution of 0.5M iron nitrate (Fe(NO3)3) and cobalt nitrate (Co(NO3)2) (Fe:Co molar ratio 1:1), stirred at 60°C for 2 h; after drying, heated to 700°C at a rate of 3°C / min under argon protection, carbonized for 1 h to form a metal oxide layer, and a graphite negative electrode material is obtained.

[0109] Example 2

[0110] In this example, a graphite negative electrode material is prepared, and the specific steps are as follows:

[0111] 1) Dopamine and polyethylene fibers with a molecular weight of 3 x 10 6 g / moL are mixed according to a mass ratio of 1:3 and dissolved in 200 mL of deionized water, stirred at a temperature of 60°C for 4 h, and an intermediate material is obtained;

[0112] 2) 80 g of artificial graphite is dispersed in 200 mL of hydrochloric acid solution with a pH value of 3.0, then mixed with the intermediate material according to a ratio of 1:1, vacuum dried, and heated to 500°C at a rate of 3°C / min in a nitrogen atmosphere, pyrolyzed for 2 h to form a hard carbon layer, and a graphite precursor is obtained;

[0113] 3) The graphite precursor is immersed in a mixed solution of 0.6 M nickel nitrate (Ni(NO3)2) and iron nitrate (Fe(NO3)3) (Fe:Ni molar ratio 1:2) and stirred at 60°C for 2 h. After drying, the temperature is raised to 700°C at a rate of 3°C / min under argon, and carbonization is performed for 1.5 h to form a metal oxide layer, thereby obtaining the graphite negative electrode material.

[0114] Example 3

[0115] In this example, a graphite negative electrode material is prepared according to the following specific steps:

[0116] 1) Dopamine and polystyrene fibers with a molecular weight of 3 x 10 6 g / moL are mixed at a mass ratio of 2:3 and dissolved in 200 mL of deionized water, and stirred at 70°C for 4 h to obtain an intermediate material;

[0117] 2) 50 g of natural flake graphite is dispersed in 200 mL of a hydrochloric acid solution with a pH value of 2.5, and then mixed with the intermediate material at a ratio of 1:1. After vacuum drying, the temperature is raised to 400°C at a rate of 3°C / min under a nitrogen atmosphere, and pyrolysis is performed for 2 h to form a hard carbon layer, thereby obtaining a graphite precursor;

[0118] 3) The graphite precursor is immersed in a mixed solution of 0.6 M nickel nitrate (Ni(NO3)2) and iron nitrate (Fe(NO3)3) (Fe:Ni molar ratio 1:2) and stirred at 60°C for 2 h. After drying, the temperature is raised to 700°C at a rate of 3°C / min under argon, and carbonization is performed for 1.5 h to form a metal oxide layer, thereby obtaining the graphite negative electrode material.

[0119] Example 4

[0120] In this example, a graphite negative electrode material is prepared according to the following specific steps:

[0121] 1) Dopamine and polystyrene fibers with a molecular weight of 3 x 10 7 g / moL are mixed at a mass ratio of 2:3 and dissolved in 200 mL of deionized water, and stirred at 70°C for 4 h to obtain an intermediate material;

[0122] 2) 50 g of natural flake graphite is dispersed in 200 mL of a hydrochloric acid solution with a pH value of 2.5, and then mixed with the intermediate material at a ratio of 1:1. After vacuum drying, the temperature is raised to 400°C at a rate of 3°C / min under a nitrogen atmosphere, and pyrolysis is performed for 2 h to form a hard carbon layer, thereby obtaining a graphite precursor;

[0123] 3) The graphite precursor was immersed in a 0.4 M ferric chloride (FeCl3) solution, stirred at 60 °C for 2 h; after drying, it was carbonized at 700 °C for 1.5 h under argon at a heating rate of 3 °C / min to form a metal oxide layer, obtaining the graphite anode material.

[0124] Example 5

[0125] In this example, a graphite anode material was prepared, and the specific steps were as follows:

[0126] 1) Dopamine was mixed with polyimide fibers with a molecular weight of 3 x 10 6 g / moL in a mass ratio of 1:2 and dissolved in 200 mL of deionized water, stirred at a temperature of 60 °C for 4 h to obtain an intermediate material;

[0127] 2) 50 g of natural flake graphite was dispersed in 200 mL of a hydrochloric acid solution with a pH value of 2.5, then mixed with the intermediate material in a 1:1 ratio, vacuum dried, and pyrolyzed at 300 °C for 2 h under a nitrogen atmosphere to form a hard carbon layer, obtaining a graphite precursor;

[0128] 3) The graphite precursor was immersed in a mixed solution of 0.5 M ferric acid (Fe(NO3)3) and cobalt nitrate (Co(NO3)2) (Fe:Co molar ratio 1:1), stirred at 60 °C for 2 h; after drying, it was carbonized at 600 °C for 2 h under argon at a heating rate of 3 °C / min to form a metal oxide layer, obtaining the graphite anode material.

[0129] Comparative Example 1

[0130] The difference between this comparative example and Example 1 is that dopamine in step 1) is replaced by phenol-formaldehyde resin.

[0131] Comparative Example 2

[0132] The difference between this comparative example and Example 1 is that step 2) is not performed, and the intermediate material is directly used in step 3).

[0133] Comparative Example 3

[0134] The difference between this comparative example and Example 1 is that step 3) is performed first, followed by step 2), i.e., step 3) is performed first using the intermediate material, and then step 2) is performed to form a hard carbon layer on the surface of the metal oxide layer.

[0135] Comparative Example 4

[0136] The difference between this comparative example and Example 1 is that the pyrolysis temperature in step 2) is 600 °C, and the pyrolysis time is 2 h.

[0137] Comparative Example 5

[0138] The difference between the present comparative example and Example 1 is that the temperature of pyrolysis in step 2) is 200℃ and the pyrolysis time is 2h.

[0139] Comparative Example 6

[0140] The difference between the present comparative example and Example 1 is that there is no step 3), i.e. the graphite precursor material in step 2) is the final product.

[0141] Comparative Example 7

[0142] The difference between the present comparative example and Example 1 is that no polyethylene fiber is used in step 1), and dopamine is directly dissolved in deionized water, stirred at a temperature of 60℃ for 4h to obtain the intermediate material.

[0143] Test Example 1

[0144] The N2 adsorption-desorption specific surface area of the graphite negative electrode materials of Examples 1-5 and Comparative Examples 1-7 is detected. The specific surface area is determined by gas adsorption method, and the interlayer spacing is calculated by XRD, with the incident light wavelength being The cross-section is tested by TEM to determine the thickness of the coating layer. The results are shown in Table 1.

[0145] The SEM images of the graphite negative electrode material prepared in Example 1 are shown in Figure 2 and Figure 3 The SEM images of the graphite negative electrode material prepared in Example 2 are shown in Figure 4 and Figure 5 . Among them, Figure 2 the magnification is 3000x, Figure 3 is a partial enlarged view of Figure 2 , the magnification is 10000x, among them, Figure 4 the magnification is 3000x, Figure 5 is a partial enlarged view of Figure 4 , the magnification is 10000x.

[0146] Table 1

[0147]

[0148] From Table 1 of Examples 1-5 and Comparative Examples 1-7, it can be seen that by covering the graphite surface pores with hard carbon / metal oxide, the specific surface area of the prepared graphite negative electrode material is significantly reduced, and the hard carbon / metal oxide fills the gap between the graphite particles, so that the compaction density of the prepared graphite negative electrode material is improved. Moreover, due to the disorder of the surface coated hard carbon, the interlayer spacing of the prepared graphite negative electrode material is expanded, and after high-temperature carbonization, the microcrystalline structure of the surface coating layer grows, which can effectively improve the rate performance.

[0149] Test Example 2

[0150] The graphite negative electrode materials obtained in Examples 1-5 and Comparative Examples 1-7 were used as negative electrode materials, and lithium ion batteries were prepared according to the following methods. The preparation method of the button lithium ion battery comprises the following steps:

[0151] According to the ratio of active material: SP: CMC: SBR = 94.5: 1.5: 1.5: 2.5, the negative electrode material, SP, CMC and SBR were weighed and uniformly mixed in deionized water to prepare a slurry; the uniformly mixed slurry was coated on an aluminum foil current collector, and was baked in an 80°C oven for 1 h, and then was cooled to room temperature.

[0152] The electrode sheet was cut to prepare a small disc with a diameter of 14 mm and weighed as m1, and the aluminum foil current collector was also cut to prepare an aluminum foil disc with a diameter of 14 mm and weighed as m2. The mass of the active material was (m1-m2)*0.945, denoted as m3. The weighed small disc was placed in a 80°C oven for vacuum baking for 12 h.

[0153] The vacuum-baked small disc was transferred to a glove box, lithium pieces were used as the counter electrode and auxiliary electrode, the electrolyte was 1M LiPF6 / EC:DEC = 1:1, and glass fiber separators were used as separators to assemble lithium ion button batteries in a glove box with oxygen and water content less than 0.01 ppm. The assembled button lithium ion batteries were allowed to stand for 12 h. The button lithium ion batteries were tested for electrochemical performance on a Wuhan Blue Electric Battery Test System. The test results are shown in Table 2.

[0154] Table 2

[0155]

[0156] As can be seen from Table 2, the specific capacity and the first coulombic efficiency of the prepared graphite negative electrode can be effectively improved by coating the hard carbon / metal oxide, and the capacity retention rate under 1C cycling can be effectively improved by coating the surface with metal oxide to improve the ion transmission speed and improve the rate performance. By coating the surface with hard carbon, the specific surface area of the graphite is reduced, and a stable surface-coated hard carbon structure is formed by pyrolysis and carbonization at a certain temperature, which is beneficial to the subsequent attachment of metal salt.

[0157] Obviously, the above examples are only examples for clearly illustrating the present application, and are not intended to limit the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, it is not necessary or possible to exhaust all embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A graphite negative electrode material, characterized in that The invention comprises graphite, a hard carbon layer coated on the surface of the graphite, and a metal oxide layer coated on the surface of the hard carbon layer; Wherein, the graphite negative electrode material satisfies the relationship: 0.5 1.5, represents the carbon layer spacing of the graphite negative electrode material, represents the compaction density of the graphite negative electrode material, represents the specific surface area of ​​the graphite negative electrode material, represents the average thickness of the hard carbon layer, represents the average thickness of the metal oxide layer; The method of the graphite negative electrode material comprises the following steps: S1: reacting dopamine with a high-performance fiber, so that the dopamine polymerizes on the surface of the high-performance fiber and forms an adhesive film, thereby obtaining an intermediate material; S2: mixing the intermediate material with graphite so that the graphite is coated with the intermediate material, and then pyrolyzing the mixture to obtain a graphite precursor; wherein the pyrolysis temperature is 300-500° C.; S3: Immersing the graphite precursor in a metal salt solution, drying, and then carbonizing at a low temperature to obtain the graphite negative electrode material.

2. The graphite negative electrode material according to claim 1, characterized in that The graphite negative electrode material meets at least one of the following conditions: a) the thickness ratio of the hard carbon layer to the metal oxide layer is (1.3-3.6):1; b) the average thickness of the hard carbon layer is 60 to 90 nm; c) the average thickness of the metal oxide layer is 25 to 45 nm; d) The hard carbon layer is formed by carbonizing high-performance fibers, and the high-performance fibers are high-performance fibers polarized with polydopamine.

3. The graphite negative electrode material according to claim 1, characterized in that The graphite negative electrode material meets at least one of the following conditions: e) Carbon interlayer spacing of the graphite negative electrode material 0.355~0.361nm; f) The specific surface area SSA of the graphite negative electrode material is ≤ 2.5 m 2 / g; g) The compaction density of the graphite negative electrode material is ρ≥1.55 g / cm 3 ; h) The average width L of the graphite crystals in the graphite negative electrode material along the a-axis direction a 75~100nm; i) The average height L of the graphite crystals in the graphite negative electrode material along the c-axis direction c 25~35nm.

4. The graphite negative electrode material according to claim 2, characterized in that The high-performance fibers include at least one or more of polyethylene fibers, polystyrene fibers and polyimide fibers.

5. A method for preparing the graphite negative electrode material according to any one of claims 1 to 4, characterized in that: The steps include: S1: reacting dopamine with a high-performance fiber, so that the dopamine polymerizes on the surface of the high-performance fiber and forms an adhesive film, thereby obtaining an intermediate material; S2: mixing the intermediate material with graphite so that the graphite is coated with the intermediate material, and then pyrolyzing the mixture to obtain a graphite precursor; S3: Immersing the graphite precursor in a metal salt solution, drying, and then carbonizing at a low temperature to obtain the graphite negative electrode material.

6. The method according to claim 5, characterized in that Step S1 includes: The dopamine and the high-performance fiber are dissolved in water and stirred at a temperature of 60-80° C. for 2-5 hours, so that the dopamine is polymerized on the surface of the high-performance fiber to form an adhesive film, thereby obtaining an intermediate material.

7. The method according to claim 6, characterized in that The high-performance fibers include at least one or more of polyethylene fibers, polystyrene fibers and polyimide fibers.

8. The method according to claim 6, characterized in that The molecular weight of the high performance fiber is ≥3×10 6 g / moL.

9. The method according to claim 6, characterized in that The mass ratio of the dopamine to the high-performance fiber is 1:(1-3).

10. The method according to claim 5, characterized in that Step S2 includes: Dispersing the graphite in a solution with a pH of 2.0 to 3.0 to obtain a graphite dispersion solution; and The graphite dispersion solution and the intermediate material are stirred and mixed so that the graphite is coated with the intermediate material, and then dried and pyrolyzed in an inert atmosphere at 300-500° C. for 2-4 hours to obtain the graphite precursor.

11. The method according to claim 10, characterized in that The average particle size of the graphite is 8-20 μm.

12. The method according to claim 10, characterized in that The solution is one or more of hydrochloric acid, nitric acid, sulfuric acid and HF.

13. The method according to claim 10, characterized in that The stirring and mixing time is 30~90min.

14. The method according to claim 10, characterized in that The concentration of the graphite dispersion solution is 0.2-0.4 kg / L.

15. The method according to claim 10, characterized in that The inert atmosphere in step S2 is one or more of helium, nitrogen, and argon.

16. The method according to claim 5, characterized in that Step S3 includes: The graphite precursor is immersed in the metal salt solution, dried, and then carbonized in an inert atmosphere at a temperature of 500-700° C. for 1-2 hours to obtain the graphite negative electrode material.

17. The method according to claim 16, characterized in that The metal salt solution is one or more of aluminum nitrate, ferric nitrate, ferric chloride and nickel nitrate solution.

18. The method according to claim 16, characterized in that The inert atmosphere in step S3 is one or more of helium, nitrogen, and argon.

19. A negative electrode sheet, characterized in that: The invention comprises the graphite negative electrode material according to any one of claims 1 to 4, or the graphite negative electrode material prepared by the method according to any one of claims 5 to 18.

20. A lithium ion battery, characterized in that: Including the negative electrode sheet according to claim 19.

21. The lithium-ion battery according to claim 20, characterized in that The lithium-ion battery has a reversible discharge specific capacity of ≥375 mAh / g, an initial coulombic efficiency of ≥95%, a capacity retention rate of ≥90% after 200 cycles at 1C, and a capacity retention rate of ≥75% at 5C.

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